Multiple antenna techniques, also known as MIMO (multiple-input multiple-output) are commonly used in wireless communications systems. A general block diagram of a MIMO system is illustrated in FIG. 1. The system may comprise a transmitter 10 and corresponding receiver 11. The transmitter and receiver may also comprise multiple transmit and receive antennae as shown.
Multiple antennae at the transmitter and receiver can be used for different purposes which may include increasing the data throughput, improving performance over fading channels, or both. One of the most common MIMO techniques is the so-called spatial multiplexing (SMX), which comprises transmitting parallel independent data streams from the different transmit antennae. Compared to a single-input single-output (SISO) system, an SMX system with two transmit antennae doubles the system throughput, and similarly, it quadruples the system throughput when four transmit antennae are present as would be understood.
Recently, a class of MIMO systems, referred to as Spatial Modulation (SM), has been introduced with the purpose of reducing the number of RF chains in the transmitter and thus reducing the overall transmission system complexity.
FIG. 2A illustrates the concept of SM (where two antennae may be driven by the same transmitter or by different transmitters). Consider a pair of symbols (s1, s2) of an incoming serial data stream 20. As shown in FIG. 2A, spatial multiplexing transmits symbol s1 from the first antenna 23 (referred to hereinafter as Tx1) and symbol s2 from the second antenna 24 (referred to hereinafter as Tx2). In spatial modulation, illustrated in FIG. 2B, only one of the two transmit antennae is active at any one time, and the active antenna is determined by an information bit 21. In the example of FIG. 2B (where one transmitter drives both antennae in turn), Tx1 is active when information bit 21 is 0, and Tx2 is active when the information bit is 1. To simplify the description of SM, suppose that the modulation is binary phase-shift keying (BPSK), in which each symbol carries a single bit as would be understood. Symbol s2 is used to select the antenna from which symbol s1 will be transmitted. As shown in FIG. 2B, symbol s1 is transmitted from Tx1 when s2=0. To summarize, SM transmits two bits per channel use (bpcu) with BPSK (1 bit transmitted by the modulation and one bit determined by the antenna that was used to transmit), three bpcu with quaternary phase shift keying (QPSK), five bpcu with 16-state quadrature amplitude modulation (16-QAM), and more generally, m+1 bpcu with a modulation, which has M=2m constellation points. In comparison, SMX (FIG. 2A) has 2m bpcu capacity, because it can transmit two symbols in parallel.
Both the systems of FIGS. 2A and 2B may comprise a processor with associated support components, some being RF support components for providing the antenna drive signals and for manipulating the incoming data stream into transmittable symbols as would be understood.
For SM with four transmit antennae, the number of transmitted bits per channel use becomes m+2, because two bits are assigned to the selection of one active antenna among four. There are also other variants of SM in which more than one antenna transmits simultaneously. For example, if there are four transmit antennae and two of them are active (they transmit simultaneously), SM transmits 2m+2 bpcu, because two bits are assigned to select the active antennae and two symbols (of m bits each) are transmitted from the selected two antennae.
With the same number of antennae, SMX transmits 4m bpcu. Therefore the reduced number of RF chains in the transmitter and the reduced complexity of SM come at the expense of reducing the data throughput compared to known SMX systems.
As a further example, consider a conventional SM system with two transmit antennae and using QPSK modulation. This system is illustrated in Table I below, which shows the two combinations, denoted C1 and C2.
TABLE IConventional SM with 2TxTx1Tx2C1QPSK0C20QPSK
Combination C1 comprises activating the first antenna (Tx1) and transmitting a QPSK symbol from that antenna. Similarly, combination C2 comprises activating the second antenna (Tx2) and transmitting a QPSK symbol from that antenna. As can be seen, the total number of combinations is two, therefore one bit is sufficient to select a particular combination. As is known, two bits determine a particular QPSK symbol to be transmitted from the selected antenna. Therefore, this conventional scheme transmits three bpcu.
There is, therefore, a need to increase the data throughput whilst maintaining the use of a less complex system as with conventional SM.